This invention relates to techniques for sealing flat-panel devices such as flat-panel displays.
A flat-panel device typically contains two generally flat plates positioned opposite each other. A flat-panel display is a type of flat-panel device utilized for displaying information. The two plates in a flat-panel display are commonly termed the faceplate and backplate. The faceplate, which provides the display""s viewing surface, is part of a faceplate structure containing one or more layers or regions formed over the faceplate. The backplate is similarly part of a backplate structure containing one or more layers or regions formed over the backplate. The two plate structures are sealed together, typically through an outer wall, to form a sealed enclosure.
A flat-panel display utilizes mechanisms such as cathode rays (electrons), plasmas, and liquid crystals to display information on the faceplate. Flat-panel displays which employ these three mechanisms are generally referred to as cathode-ray tube (xe2x80x9cCRTxe2x80x9d) displays, plasma displays, and liquid-crystal displays. The constituency and arrangement of the display""s two plate structures depend on the type of mechanism utilized to display information on the faceplate.
In a flat-panel CRT display, electron-emissive elements are typically provided over the backplate. Light-emissive elements are situated over the faceplate. When the electron-emissive elements are appropriately excited, they emit electrons that strike the light-emissive elements causing them to emit light visible on the faceplate. By appropriately controlling the electron flow from the backplate structure to the faceplate structure, a suitable image is displayed on the faceplate. The electron flow needs to occur in a highly evacuated environment for the CRT display to operate properly and to avoid rapid degradation in performance. It is thus critical to hermetically seal a flat-panel CRT display.
FIGS. 1a-1c (collectively xe2x80x9cFIG. 1xe2x80x9d) illustrate a conventional technique for sealing a flat-panel CRT display of the field-emission type, often referred to simply as a field-emission display (xe2x80x9cFEDxe2x80x9d). The components of the FED being sealed in FIG. 1 include backplate structure 10, faceplate structure 12, outer wall 14, and multiple spacer walls 16 situated between plates structures 10 and 12 for preventing outside forces, such as air pressure, from collapsing or otherwise damaging the FED.
At the point shown in FIG. 1a, spacer walls 16 are mounted on faceplate structure 12, and outer wall 14 is connected to faceplate structure 12 through frit (sealing glass) 18 provided along the faceplate edge of outer wall 14. Frit 20 is situated along the backplate edge of outer wall 14. A pump-out tube (not shown) is typically affixed to backplate structure 10 for later evacuating the sealed FED. Prior to the sealing operation, backplate structure 10 is physically separate from the composite structure formed with faceplate structure 12, outer wall 14, and spacer walls 16.
Structures 10 and 12/14/16 are placed in an alignment system 22, aligned to each other, and brought into physical contact along frit 20 as shown in FIG. 1b. Alignment system 22 is located in, or is placed in, an oven 24. After being aligned and brought into contact along frit 20, structures 10 and 12/14/16 are slowly heated in air to a sealing temperature ranging from 450xc2x0 C. to greater than 600xc2x0 C. Frit 20 melts. The FED is subsequently cooled down to room temperature. As frit 20 cools down, it seals composite structure 12/14/16 to backplate structure 10.
At or near the end of the cooldown, the FED is removed from alignment system 22 and oven 24. The pressure in the interior of the FED is brought down to the desired vacuum level by removing air through the pump-out tube. The pump-out tube is then closed. Aside from the pump-out tube, FIG. 1c depicts the final hermetically sealed FED.
During the sealing operation, the upper edge of outer wall 14, including frit 18 and frit 20, is initially slightly higher than the upper edges of spacer walls 16. As frit 20 melts, it compresses somewhat in the direction, commonly referred to as the z direction, perpendicular to plate structures 10 and 12 until spacer walls 16 meet backplate structure 10. Frit 18 may also compress in the z direction during the sealing operation. Hence, plates structures 10 and 12 move relative to each other in the z direction as the FED is being sealed. A similar type of z motion would occur if a rectangular ring of frit were substituted for composite outer wall 14/18/20.
A side effect of motion in the z direction is that faceplate structure 12 sometimes moves relative to backplate structure 10 in a direction perpendicular to the z direction. Hence, the alignment of plate structures 10 and 12 is sometimes degraded as a result of the z motion of structures 10 and 12. Due primarily to differences in the coefficients of thermal expansion of plate structures 10 and 12 and alignment system 22, the degradation in alignment can occur despite the use of system 22. It would be desirable to hermetically seal a flat-panel display, especially a flat-panel CRT display such as an FED, according to a technique that largely avoids z motion between the displays two plate structures and thus avoids alignment degradation due to such z motion.
As frit 20 melts and compresses in the z direction, frit 20 normally spreads laterally over faceplate structure 12. The lateral area of structure 12 can be increased in the peripheral area outside the viewing area to allow for frit 20 to spread laterally. However, it is typically desirable that the peripheral display area be as small a fraction as possible of the total lateral area of structure 12. Accordingly, increasing the lateral area of structure 12 to allow room for frit 20 to spread is disadvantageous.
In addition, frit 20 may occasionally spread laterally beyond the normal area allocated for the spreading of frit 20 and damage components of the FED. A similar disadvantage would occur if composite outer wall 14/18/20 were replaced with a ring, again rectangular, of frit. In sealing two plate structures of a flat-panel display, especially a flat-panel CRT display such as an FED, together through a sealing structure, it would be desirable to have a technique for suitably restricting lateral spreading of the sealing material in the sealing structure.
PCT Patent Publication WO 98/26440 discloses a local-energy gap-jumping technique for sealing the backplate structure and faceplate structure of a flat-panel display. A rectangular frame of sealing material, typically frit, is sealed to the faceplate structure. The sealing frame laterally surrounds a group of spacer walls that extend further away from the faceplate structure than does the sealing frame. The backplate structure is placed vertically above the faceplate structure so that the sealing frame and spacer walls are situated between the two plate structures. The backplate structure lies directly on the spacer walls. Because the spacer walls are taller than the sealing frame at this point, a gap is present between the backplate structure and the sealing frame.
The two plate structures in PCT Patent Publication WO 98/26440 are held in a desired alignment using a suitable tacking mechanism. Energy is then transferred locally to portions of the sealing frame close to the backplate structure. The local energy, typically light energy provided from a laser or focused lamp, causes the sealing material to jump the backplate-structure-to-sealing-frame gap and hermetically seal the plate structures together.
By using spacer walls that are initially taller than the sealing frame, the sealing technique of PCT Patent Publication WO 98/26440 largely avoids undesired z motion during the sealing operation. However, utilization of a laser, focused lamp, or other local-energy producing mechanism to direct energy locally onto the sealing frame can sometimes be relatively time-consuming and thus unduly expensive. It would be desirable to have a technique that can be implemented rapidly, and relatively inexpensively, to seal a flat-panel display such as an FED.
The present invention furnishes techniques for sealing a flat-panel device so as to achieve a hermetic seal while avoiding the above-mentioned disadvantages of the prior art. The sealing techniques of the invention are especially suitable for sealing a flat-panel CRT display, such as an FED, in which the interior of the display needs to be at a high vacuum during display operation. Nonetheless, each of the present sealing techniques can be applied to a display which requires a strong seal even though the display""s interior may not be at a high vacuum during display operation.
In one aspect of the invention, sealing of first and second plate structures of a flat-panel device to each other is performed under the influence of gravity. More particularly, sealing material is provided in a specified pattern over the second plate structure. The first plate structure is positioned vertically below the second plate structure so that the sealing material lies between the two plate structures. As used here in describing gravitational sealing of two plate structures, the term xe2x80x9cverticallyxe2x80x9d means vertically relative to the body, such as the earth, which provides the gravitation. The sealing material is then heated so that it moves downward under gravitational influence to contact the first plate structure and seal the plate structures together.
The plate structures are preferably maintained in a largely fixed positional relationship to each other during the heating step. For instance, the positioning of the first plate structure below the second plate structure is preferably conducted in such a way that the plate structures are spaced vertically apart from each other in largely a fixed manner. That is, the spacing between the plate structures along any vertical line through the plate structures is approximately constant. This positional relationship is then maintained during the heating step using, for example, an intermediate mechanism situated between the plate structures.
Importantly, by maintaining the plate structures in largely a fixed positional relationship to each other during the heating step, there is a essentially no z motion of one of the plate structures relative to the other during the heating step. Inasmuch as such z motion during the sealing of a pair of plate structures to each other often causes degradation in the alignment of the plate structures to each other, sealing the first and second plate structures together under the influence of gravity with the plate structures held in largely a fixed positional relationship to each other so as to avoid such z motion also avoids associated alignment degradation.
The heating step during the gravitational sealing operation preferably entails globally heating the sealing material and the two plate structures. The term xe2x80x9cglobalxe2x80x9d or xe2x80x9cgloballyxe2x80x9d as used here in describing a heating operation performed on parts of a device means that the heat is applied in a generally non-selective manner to the parts of the device. A global heating operation is thus basically the converse of a local heating operation in which energy is directed selectively to certain material largely intended to receive the energy without being significantly directed to nearby material not intended to receive the energy. Global heating is typically less time-consuming, and thus less expensive, than local heating. As a result, using global heating to perform the heating step of the present gravitational sealing operation helps keep the sealing cost down.
In another aspect of the invention, one or more restricting structures are utilized to limit the area where first and second plate structures of a flat-panel device are sealed to each other. The seal-restricting structure or structures thereby prevent the sealing material from spreading to sensitive device areas and degrading the device.
Specifically, one or two seal-restricting structures are provided over the first plate structure. Sealing material is provided in a specified pattern over the second plate structure. The plate structures are then positioned generally opposite each other so that the sealing material and the restricting structure or structures lie between the plate structures. If only one restricting structure is provided over the first plate structure, the sealing material is situated opposite a location close to the restricting structure. When two restricting structures are provided over the first plate structure, the sealing material is situated opposite a location between the restricting structures.
The sealing material is heated to seal the plate structures together. If one restricting structure is provided over the first plate structure, the sealing material contacts the first plate structure close to that restricting structure. The restricting structure largely prevents the sealing material from spreading laterally over the restricting structures and contacting the first plate structure laterally beyond the restricting structure. When two restricting structures are placed over the second plate structure, the sealing material contacts the first plate structure between the restricting structures. The two restricting structures then largely prevent the sealing material from spreading laterally over the restricting structure and contacting the first plate structure laterally beyond one or both of the restricting structures. In either case, use of the restricting structure or structures typically prevents the sealing material from spreading laterally in such a manner as to degrade the flat-panel device. Also, the lateral area of the flat-panel device need not be significantly increased to allow for lateral spreading of the sealing material.
In a further aspect of the invention, first and second plate structures of a flat-panel device are sealed together according to a global-heating gap-jumping technique. In particular, sealing material is again provided in a specified pattern over the second plate structure. The two plate structures are then positioned opposite each other so that the sealing material lies between the plate structures. The positioning step is done in such a way that a gap separates the first plate structure from the sealing material provided over the second plate structure.
The first plate structure is preferably positioned vertically above the second plate structure. Similar to what was said above about the meaning of the term xe2x80x9cverticallyxe2x80x9d in connection with the gravitational sealing technique of the invention, the term xe2x80x9cverticallyxe2x80x9d as used in connection with the present global-heating gap-jumping technique means vertically relative to the underlying major gravitational body above which the global-heating gap-jumping technique is performed. With this in mind, the preferred orientation of the first plate structure above the second plate structure in the global-heating gap-jumping technique is opposite to the orientation in which the plate structures are arranged during the heating step of the gravitational sealing technique.
The sealing material and plate structures in the present global-heating gap-jumping technique are then globally heated to cause the sealing material to bridge the gap between the plate structures and seal them together. In the preferred case where the first plate structure is positioned vertically above the second plate structure, the sealing material provided over the second plate structure moves vertically upward to jump the gap. By using global heating to produce gap jumping, the cost of the sealing operation can be kept relatively low.
The present gravitational sealing technique can be performed with one or two seal-restricting structures. The same applies to the global-heating gap-jumping sealing technique of the invention. By maintaining the plate structures in largely a fixed positional relationship to each other during the heating step, the resultant sealing technique achieves both the advantages of using one or two seal-restricting structures and the advantages of the gravitational or global-heating gap-jumping technique. That is, device alignment degradation caused by z motion during the sealing operation is largely avoided, the sealing material is largely prevented from spreading over undesirable device areas and damaging sensitive device elements, and the device""s lateral area need not be significantly increased to accommodate spreading of the sealing material.
In short, use of the present sealing techniques enables a flat-panel device to be hermetically sealed in a manner that avoids critical degradation problems. The sealing operation can be performed in a highly cost-efficient manner. The invention thereby provides a substantial advance over the prior art.